This invention relates to an apparatus and method for protecting privacy for message signals on a telephone network. More particularly, an improved apparatus and method is disclosed which allows message signals to be exchanged between predetermined authorized telephone subscribers. This improved apparatus and method can accommodate clear or coded message signals, such as clear analog signals or digitally encrypted coded signals, and in addition, has a signaling format which is compatible with other communication networks composed of a mixture of secure and conventional equipment so that secure, end-to-end communications are permitted to take place automatically regardless of whether the message source is a similarly equipped telephone subscriber or a subscriber on another communications network, such as a radio communications network.
The trend of today's communications systems is generally towards requiring more secure communications in order to prevent unauthorized reception and detection of certain sensitive information. Most often, such unauthorized reception and detection poses a threat to message signals on a telephone network such as the Public Switched Telephone Network. However, with the greater use of radio communication networks, there is an even greater need for protecting privacy for message signals on an integrated system, such as one containing a telephone network and a radio network. In sending sensitive information throughout such a communication system, particularly a large system such as used by a Metropolitan Police Department, it is necessary that the signal remain in a secure format while it is being passed through a large number of components. If possible, a secure signal format should be maintained while the signal is being transmitted from a mobile radio to a base station receiver serving a group of mobile radios within its coverage area, as well as through audio interconnect and patching systems such as those connecting a console unit for a radio network operator or a telephone patch providing interconnect calls to a telephone user via the Public Switched Telephone Network or PSTN. Many analog voice privacy systems have been available for some time, but offer limited security because they use frequency inversion, band splitting, and other types of analog signal scrambling techniques which are easily descrambled due to the limited number of combinations possible. It has been found that a high degree of security is obtained in a digital scrambling system where a voice information signal or clear message signal is first analog-to-digital converted in an A/D converter stage. The outputted digital signals are then encoded into a scrambled form by using shift registers and various gates to produce a pseudo-randomly encoded, or scrambled signal, hereinafter referred to as a coded message signal.
Various digital scrambling systems are available, which have limited usefulness for point-to-point, single path communications such as, for example, from a mobile radio to a mobile radio, from a mobile radio to a base station, or from a base station to another base station. These systems have no facility for multi-network interconnecting. In order for a scrambled message to be transmitted throughout a system composed of a mixture of the above networks, it is often necessary to unscramble, or decrypt, the coded message signal into the original clear message signal before again encrypting. This may provide a potential access point for an intruder into an otherwise secure communication network. Furthermore, when it is desirable to change the encryptor/decryptor code keys in such a system, it is necessary to insert the new codes at each equipment site. This is an inconvenient and time-consuming procedure.
Scramblers for radio communication networks are generally considered accessories to the units in such networks, and do not readily adapt themselves to full network operation. Moreover, portable radios have not been widely adapted for use in protected communication systems. Often times, these portable radios must be used with satellite receiver voting systems in which the best received signal is selected from a plurality of receive only sites serving a base station in a central location. Digital scrambling systems are generally not compatible with the analog voting system capabilities used in some satellite receiver systems of the known art. Many known art scrambler accessories require signal bandwidths much greater than available on the telephone network or in standard radio communication equipment. These accessories cannot be used in multi-equipment linked networks having standard bandwidths and components.
Another problem in the known art is that because digital scramblers oftentimes do not use standard channel bandwidths for transmitting signals, it is not possible to effectively intermix secure and conventional equipment associated with various single communication networks into a desired integrated system. Thus, equipment designed for voice privacy on the telephone network is often incompatible with the equipment and signaling design for radio communication networks for handling both coded and clear message signals. Known art digital scrambler systems, therefore, do not lend themselves to automatic transparent operation, that is, automatic coded/clear operation in which the message signal being transmitted is properly handled by the system components regardless of whether the message signal is in a coded or clear form. This is especially true for digital voice protected telephone networks when interfacing to a radio communication network so as to include predetermined radio subscribers, since the signalling requirements are different between the two networks.
Finally, many known art digital scramblers used in communications systems consisting of a mixture of various networks are not capable of insuring secure end-to-end communications between a subscriber on one system, such as a telephone subscriber, and that on another, such as a mobile radio subscriber. In order to protect sensitive information being exchanged between subscribers on different networks, it is desirable to have the integrated system automatically determine whether a coded or clear message signal is to be exchanged, independent of which network or which subscriber initiated the call. And yet, in order to maintain compatibility with subscribers having conventional equipment, as well as for ease of operation for those predetermined subscribers having the coded or clear message signal choice, it is desirable to have the integrated system automatically detect standard signalling (such as call progress and DTMF - dual tone multi-frequency signalling) on the telephone network.
Consequently, there exists a need for an improved secure telephone terminal, or remote interface, and method which permits coded or clear message signals to automatically exchange between telephone subscribers as well as interconnecting two other communication networks with a signaling format which permits secure, end-to-end communications to take place without intervening decryption stages. This terminal apparatus and method should be compatible with conventional equipment bandwidth and signalling constraints associated with each communication network, and further should include the capability to return to a conventional connection mode when a power failure is detected within the secure telephone terminal at the locality of a given telephone subscriber.